1. Field of the Invention
The present invention relates to a measurement method, a measurement apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
In recent years, a projection optical system (projection lens) built into a projection exposure apparatus is required to have a performance high enough to suppress wavefront aberrations (transmission wavefront aberration) below 10 mλ RMS (e.g., wavelength λ=248 nm or 193 nm). This makes it necessary to measure the wavefront aberration of the projection optical system with high precision (about 1 mλ), with the projection optical system built into the exposure apparatus. Simplification, speed-up, and the like of the measurement are also important to increase productivity.
An interferometer is used to measure the wavefront aberration of a projection optical system by measuring the wavefronts of the projection optical system at a plurality of points in its field region. The projection optical system is adjusted so as to minimize an aberration coefficient obtained by the polynomial expansion (e.g., using the Zernike function) of the wavefronts measured by the interferometer.
To improve the resolution of an exposure apparatus, polarized illumination has come to be used in place of non-polarized illumination. This makes it necessary to measure not only the wavefront aberration (non-polarization wavefront) of a projection optical system with respect to non-polarized light but also its wavefront aberration (polarization wavefront) with respect to arbitrary polarized light.
Japanese Patent Laid-Open Nos. 2-116732 and 2000-071515 propose techniques of measuring the wavefront aberration of a projection optical system with respect to arbitrary polarized light. These techniques measure the birefringent characteristics of the projection optical system to obtain its wavefront aberration with respect to arbitrary polarized light using the measured birefringent characteristics. More specifically, the technique disclosed in Japanese Patent Laid-Open No. 2-116732 measures the birefringent characteristics (the magnitude and fast axis azimuth of birefringence) of the projection optical system by guiding linearly polarized light beams along four different azimuths (e.g., 0°, 90°, 180°, and 270° to it. Also, the technique disclosed in Japanese Patent Laid-Open No. 2000-071515 measures the non-polarization wavefront and birefringent characteristic (the magnitude of birefringence) of the projection optical system by guiding linearly polarized light beams along two different azimuths (e.g., 0° and 90°) to it.
Unfortunately, it is difficult for the conventional techniques to simply measure the wavefront aberration with respect to arbitrary polarized light in a short period of time. For example, Japanese Patent Laid-Open No. 2-116732 requires a relatively long measurement time because the wavefront aberrations with respect to linearly polarized light beams along four azimuths must be measured. Furthermore, Japanese Patent Laid-Open No. 2-116732 guides linearly polarized light beams along four different azimuths by rotating a λ/2 plate (polarization switching unit) inserted immediately after a light source. To maintain the linearly polarized light beams along four azimuths, the optical path (optical system) from the polarization switching unit to the projection optical system must have a non-polarization characteristic. It is, however, very hard to form such an optical path.
Japanese Patent Laid-Open No. 2000-071515 need only measure the wavefronts with respect to linearly polarized light beams along two azimuths. In this case, the measurement can be done in a short period of time, but the fast axis azimuth of birefringence must be known in advance. In addition, since a polarization switching unit is inserted immediately after a light source as in Japanese Patent Laid-Open No. 2-116732, the optical path (optical system) from the polarization switching unit to the projection optical system must have a non-polarization characteristic.